The Evolution of Antimatter Propulsion

byPaul GilsteronAugust 2, 2016

Thinking about Eugen Sänger’s photon rocket concept inevitably calls to mind his Silbervogel design. The ‘Silverbird’ had nothing to do with antimatter but was a demonstration of the immense imaginative power of this man, who envisioned a bomber that would be launched by a rocket-powered sled into a sub-orbital trajectory. There it would skip off the upper atmosphere enroute to its target. The Silbervogel project was cancelled by the German government in 1942, but if you want to see a vividly realized alternate world where it flew, have a look at Allen Steele’s 2014 novel V-S Day, a page-turner if there ever was one.

I almost said that it was a shame we don’t have a fictionalized version of the photon rocket, but as we saw yesterday, there were powerful reasons why the design wouldn’t work, even if we could somehow ramp up antimatter production to fantastic levels (by today’s standards) and store and manipulate it efficiently. Energetic gamma rays could not be directed into an exhaust stream by the kind of ‘electron gas mirror’ that Sänger envisioned, although antimatter itself maintained its hold on generations of science fiction writers and scientists alike.

Enter the Antiproton

Sänger’s presentation at the International Astronautical Congress in 1953 came just two years ahead of the confirmation of the antiproton, first observed at the Berkeley Bevatron in 1955. Now we have something we can work with, at least theoretically. For unlike the annihilation of electrons and positrons, antiprotons and protons produce pi-mesons, or pions, when they meet. Pions don’t live long, with charged pions decaying into muons and muon neutrinos, while neutral pions decay into gamma rays. Those charged pions, however, turn out to be helpful indeed.

By the early 1980s, Robert Forward had realized that superconducting coils could be used to channel such charged pions, producing the kind of directed exhaust stream that so frustrated Sänger. Forward described the ‘magnetic nozzle’ in his book Mirror Matter (Wiley, 1988), but his first paper on the subject appeared in 1982, along with other papers on antimatter propulsion from Brice Cassenti and David Morgan, all of these in the fecund pages of the Journal of the British Interplanetary Society. Morgan (Lawrence Livermore National Laboratory) envisioned a three-meter nozzle using magnetic coils to channel charged pions, with an exhaust velocity fully 94 percent of the speed of light.

The gamma ray problem is still there, but in these designs, they appear in the exhaust well behind the rocket, even as the energy of the charged pions is used to heat a propellant like hydrogen or water which becomes the exhaust. Using methods like these, we could extract up to 50 percent of the energy unlocked by the annihilation of protons and antiprotons.

In broader terms, what Forward was proposing was to replace tons of chemical propellant with milligrams of matter. In a study he performed for the Air Force Rocket Propulsion Laboratory, he advocated the creation of facilities specifically dedicated to producing antimatter, as opposed to relying on the production of antimatter in particle accelerators as a by-product of other work. In this way, he believed, the cost could be brought down to about $10 million per milligram. Remember that a milligram of antimatter produces about the same energy as 20 tons of chemical fuel, making antimatter at this price level a better deal than chemical propulsion.

We’re not at Sänger-esque levels of specific impulse (3 X 107 seconds), but Giovanni Vulpetti was able to show later in the 1980s that a rocket working with the pions of proton/antiproton annihilation was capable of a specific impulse of 0.58c. Even so, antimatter’s numerous problems continue to bedevil us. Some of Robert Frisbee’s work in the same decade overcomes the antimatter storage issue by creating spacecraft thousands of kilometers long. These fantastic designs are rapier-thin and massive, hardly the sleek starships most science fiction has led us to expect, unless we look into SF’s most extravagant imaginings.

Image: Here’s one way of getting around those huge Frisbee rockets. This is Richard Obousy’s concept for VARIES, the Vacuum to Antimatter Rocket Interstellar Explorer System. Here the starship uses an immensely powerful laser to generate its own antimatter fuel, relying on Julian Schwinger’s work showing that electron-positron pairs can be generated out of the vacuum of space itself. Credit: Adrian Mann.

Excuse the digression, but Frisbee’s work calls up the memory of one of Paul Linebarger’s stranger stories. Writing as Cordwainer Smith, Linebarger created a short fable called “Golden the Ship Was – Oh! Oh! Oh!,” which ran in Amazing Stories in April of 1959. And just as Frisbee’s vast designs stretch physics to the limit by way of showing how unlikely an antimatter starship is at our current level of understanding, Linebarger’s golden ship is in most ways a chimera, although the powers threatening the Earth do not understand what they are seeing:

“That one ship is ninety million miles long, Your Highness. It shimmers like fire, but moves so fast that we cannot approach it. But it came into the center of our fleet almost touching our ships, stayed there twenty or thirty thousandths of a second. There it was, we thought. We saw the evidence of life on board: light beams waved: they examined us and then, of course, it lapsed back into nonspace. Ninety million miles, Your Highness. Old Earth has some stings yet and we do not know what the ship is doing.”

The pleasures of Cordwainer Smith are likewise vast and I won’t give anything more away about this short tale (you can find it reprinted in The Rediscovery of Man (NESFA Press, 1993). But back to proton/antiproton annihilation, which gets an interesting new wrinkle in the work of Friedwardt Winterberg. As examined in these pages by Adam Crowl (see Re-Thinking the Antimatter Rocket), Winterberg looks at how a plasma made of matter and antimatter in equal parts (an ‘ambiplasma’) can undergo extreme compression. Let me quote Crowl:

Essentially what Winterberg describes is generating a very high electron-positron current in the ambiplasma, while leaving the protons-antiprotons with a low energy. This high current generates a magnetic field that constricts rapidly, a so-called pinch discharge, but because it is a matter-antimatter mix it can collapse to a much denser state. Near nuclear densities can be achieved, assuming near-term technical advancements to currents of 170 kA and electron-positron energies of 1 GeV.

What we get is a gamma ray flux that is highly directional, forming a gamma-ray laser, a beam of gamma rays that, in conjunction with the annihilation chamber’s magnetic fields, produces thrust. The work draws on Winterberg’s thinking on deuterium fusion rockets (he was a key contributor to the original Project Daedalus starship design) and the magnetic compression of ions. Here we get a concept that would surely have delighted Eugen Sänger, as it provides for a highly directional gamma ray thrust that was the cornerstone of the photon rocket.

All these concepts assume substantial production of antimatter and major breakthroughs in storage, but in the nearer term, we will continue to explore antimatter’s possibilities in catalyzing nuclear fusion reactions, or intriguing spacecraft designs like Steven Howe’s ‘antimatter sail.’ In Howe’s work for NASA’s original Institute for Advanced Concepts, small amounts of antimatter are used to create fission as they encounter a sail impregnated with uranium. For more, see An Antimatter Driven Sail to the Kuiper Belt.

As we learn more about storage, and in particular methods involving stable antihydrogen (a positron and an antiproton), we can hope that methods of antimatter production, and even antimatter collection in the outer Solar System, will become better understood. It will take experimentation with tiny amounts of antimatter to help us understand its possible contribution to deep space exploration.

Frisbee’s design of thousands of kilometers can be reduced by today’s technology by about a factor of a thousand down to a few kilometers. The difficulty is such a ship would have a length to diameter (L/D) ratio in the hundreds if not thousands, well above any known research.

As a sequel Marshall Eubanks has posited a very convenient source of antimatter in Near Earth Objects, in the form of Compact Condensed Objects – essentially gigantic nucleii made of quark matter. By firing a ~100 MeV proton beam at one, a stream of matter-antimatter comes out.

Of course the real problem is storing antimatter. The safest option seems to be as ultra-cold anti-hydrogen snowballs levitated by magnetic fields. I suspect once we have enough to experiment with, we might find easier ways of doing so. Theory quickly becomes bogged down in uber-complex computations when we’re talking about the real behaviour of large numbers of particles, so experiment is still needed to lead the way.

Once we have the stuff, but don’t have the unobtainium super-conductors that Winterberg’s scheme requires, I think the most effective use is as a fission catalyst. A single anti-proton can cause multiple fissions, thus an anti-proton beam could be used as the “spark plug” for a high efficiency fission-fragment rocket.

If the problem of efficient energy mixing can be solved, then Ram-Augmented Interstellar Rockets would be a highly effective use of antimatter. If the energy from 1 anti-proton+proton reaction can be effectively mixed with 400 other protons, etc., scooped from the ISM, then an exhaust velocity of ~0.1 c can be produced. RAIRs, like Ramjets proper, do require energy return from the scooped ISM to the exhaust stream, else their performance is limited by drag losses. Assuming that particular problem is solved, then the top speed gets really interesting.

How far would the “exhaust” of an anti-matter-powered gamma ray laser extend? With many of these more exotic methods of generating thrust, it seems one would need to be extremely careful where one’s back end is pointing. Or, put more simply, this sounds more like a weapon.

I cannot visualize how does an uranium-doped sail generate any thrust. With a thin sail, there would be almost as much recoil forward as it is backward, beam-wise. It is basically an energy source rather than propulsion.

I believe fission fragments generated deeper than a micron or two do not make it to the surface, at least not with any of their original momentum left. So, a thin layer of active material backed by a thin layer of inert material will allow fragments to exit only on the active side. Efficiency can never be great: Half of the fragments are lost to begin with, of the other half many go sideways too much, and in addition there will be many that partially or fully thermalize before they emerge at the surface. I think a few percent of the ideal fission fragment rocket is the best we can hope for with these sails. It may still compare favorably with nuclear electric propulsion, because of higher power density, but I wouldn’t count on that.

The major obstacles involved in antimatter technology are safe storage and, of course, efficient cost effective production of more than just minute amounts. These obstacles seem like technological in nature as opposed to physics defying obstacles, fortunately. I still think antimatter catalyzed fusion propulsion will be developed before the full-fledged, pure antimatter rockets will put into use.

mmmm….there may be a way to control the gamma ray problem by using a relativistic reaction.

If we accelerate a clump of AM to a high relativistic velocity via a particle accelerator out the back of the space craft and then very shortly after send a clump of normal matter at a higher velocity so that the normal mater catches up the AM in the reaction chamber. The ‘reaction’ will be travelling at a high c fraction and the reaction chamber walls will see the reaction gamma ray emission products as red shifted light, say to optical or any other light wavelength to be reflected by the chamber walls.

Perhaps we could possibly use a reaction chamber in the shape of many shallow angled cones which reflect gamma rays by shallow angle reflection towards the apex of the cones. At the bottom of each cone there is a very high amount of electrons which act as an electron mirror reflecting the gamma rays back out again creating impulse.

I followed your link to Steven Howe’s proposal for using a small antimatter cloud and a “sail” embedded with uranium. It seems in some ways akin to the Icarus ship, but with more, smaller “explosions” right on the structure of the ship and in front rather than behind. The antimatter sail ship as presented would surely be a lethal radiation environment for anything alive and maybe for electronics. But what about a structure with several of those sails way out to the sides? As few as two, but for added reliability I’d think six or more so if one failed steering could be maintained by the rest until repairs could be made. And what about each having a continuous roll of the “sail”, with a mechanism ahead of the thrust-producing area to re-implant uranium, if that’s needed?

Almost certainly beyond the bounds of the physically realizable, but what would happen if an antiproton reacted not with a proton but with a Delta+ baryon, the 3/2 spin analogue of the proton? How would the decay modes differ?

Am I wrong, or is it so that a proton – anti proton AM ‘engine bell’ and a magneto-inertial confinement fusion ‘engine bell’ work so much the same way that it is be possible to design an engine that could use either fuel, a kind of a ‘variable cycle’ thing?

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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